1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle, especially a plug-in electric vehicle.
2. Description of the Prior Art
In a hybrid electric vehicle equipped with a powersplit transmission, the traction motor and motor-generator are driveably connected to the driven vehicle wheels. Hence, the speed of the traction motor is always proportional to the wheel speed, and motor-generator speed is proportional to wheel speed and engine speed. The generator speed is highest, for a given vehicle speed, when the engine is off.
Similarly, the speed of the planet pinion is also proportional to the vehicle speed, and engine speed and is highest, for a given vehicle speed, when the engine is off.
Hence, the maximum allowable planetary pinion speed & generator speed determine the maximum allowable electric vehicle drive speed.
When a plug-in hybrid electric vehicle operates in charge-depleting mode, energy flows from an electric storage battery through the traction motor to the vehicle wheels. Hence, the generator spins freely. However, the ability to start the engine and provide power from the engine needs to be maintained. Hence, there is a limitation on the peak electric vehicle drive speed due to conditions described above. When the speed is higher than the peak electric vehicle drive speed, the engine must be turned on and run when it is not required.
When the vehicle is being driven in electric vehicle (EV) mode, all the elements such as generator & planetary gear set are rotating and causing additional losses.